From 3-3 in the first set, Duque-Mariño won seven straight games en route to a 63 63 victory over the No.2 seed and World No.18 Stephens. Her previous best win came over World No.26 Anna Chakvetadze at the 2009 French Open.
"I was prepared for a hard match," Duque-Mariño said. "You never know what the score is going to be, but the match was much tougher than what the score indicated."
This isn't the first landmark moment the Bogotá native and resident has had at this event, as Duque-Mariño captured her lone WTA title here in 2010.
"I'm so happy," Duque-Mariño said. "Not just for beating Stephens but also because I played really well. I think from the start of the match, I was really confident and it showed during the match."
"I'm glad that I won today," Begu said. "The conditions are very difficult with the altitude. Fortunately, I arrived a week early and played an ITF event in Medellín, Colombia, which has helped my game. The rain delay also helped. She was up 3-2 when the rain caused our match to stop. I was able to come back out on the court focused, and then I won the next several games."
Also moving on were No.5 seed Caroline Garcia and No.7 seed and 2013 Bogotá runner-up Paula Ormaechea. Next up for Ormaechea is 2012 Bogotá champion Lara Arruabarrena, who needed just 45 minutes for a 61 60 rout of María Herazo González.